In the related art, light up-conversion luminescent substances that convert long-wavelength light to short-wavelength light are known. The light up-conversion luminescent substances and inorganic light up-conversion luminescent substances in which a rare-earth element or the like is used are known. The inorganic light up-conversion luminescent substances have been applied, for example, to IR cards that convert infrared laser light to visible light, and have already been brought into practical use.
On the other hand, when inorganic light up-conversion luminescent substances in which an organic compound is used, it is known that an intense and broad absorption spectrum of the organic compound is used to enable light up-conversion with broader wavelengths and lower incident power compared with the inorganic light up-conversion luminescent substances. An example of application of the organic light up-conversion luminescent substances is organic solar cells. In an organic solar cell, those generating free charge carriers from solar light are ultraviolet light and blue light. Hence, it is expected that the photoelectric conversion efficiency of the organic solar cell will improve by using an organic light up-conversion luminescent substance in the organic solar cell to convert long-wavelength light such as green and red light into short-wavelength light such as blue light. Thus, the organic light up-conversion luminescent substances have received attention in recent years (see, e.g., Patent Document 1, Non-Patent Documents 1 and 2).
The organic light up-conversion luminescent substance is generally used together with a photosensitizer, and is used as an organic light up-conversion material. An example of a mechanism of light up-conversion in a currently known organic light up-conversion material includes the following mechanism. First, a photosensitizer molecule (1A) in its ground state absorbs light energy to transit to an excited singlet state (1A*) (1A+hν→1A*). Next, intersystem crossing to an excited triplet state (3A*) rapidly occurs (1A*→3A*), and energy is transferred from the photosensitizer molecule in the excited triplet state to a luminescent molecule. As a result, the photosensitizer molecule loses the energy to return into its ground state. On the other hand, an luminescent molecule (1E) in its ground state changes into an excited triplet (3E*) (triplet-triplet energy transfer: 3A*+1E→1A+3E*). When the concentration of luminescent molecules having changed into the excited triplet state increases, interaction between the luminescent molecules having changed into the excited triplet state occurs more efficiently, and energy transfers from the one luminescent molecule having changed into the excited triplet state to the other luminescent molecule. At this time, the one luminescent molecule having changed into the excited triplet state returns to the ground state, and the other changes into an excited singlet state (triplet-triplet annihilation process: 3E*+3E*→1E+1E*). Then, up-converted light (1E*→1E+hνf) is emitted as fluorescence from the luminescent molecule having changed into the excited singlet state. Such a mechanism is called “triplet-triplet annhilation up-conversion,” “photochemical up-conversion,” and so on.
Considering such a mechanism, in the organic light up-conversion material, it is necessary for the energy of the excited triplet state of the luminescent substance to be about half of the energy of the excited singlet state. For this reason, a luminescent substance with a molecule having an aromatic ring backbone may be used. Moreover, a photosensitizer with an organic metal complex that produces an excited triplet state with high efficiency may be used.
For example, a light up-conversion luminescent substance in a blue light emitting region, anthracene, 9,10-diphenyl anthracene and the like, are known. However, light up-conversion efficiency (i.e., conversion efficiency from long-wavelength light to short-wavelength light) using such a luminescent substance is as low as about 3 to 5%, and development of a novel organic light up-conversion material having higher light up-conversion efficiency is needed.